In thermal ink-jet printing, droplets of ink are selectably ejected from a plurality of drop ejectors in a printhead. The ejectors are operated in accordance with digital instructions to create a desired image on a print sheet moving past the printhead. The printhead may move back and forth relative to the sheet in a typewriter fashion, or the linear array may be of a size extending across the entire width of a sheet, to place the image on a sheet in a single pass.
The ejectors typically comprise capillary channels, or other ink passageways, which are connected to one or more common ink supply manifolds. Ink is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated by a heating element (essentially a resistor) disposed on a surface within the channel. This rapid vaporization of the ink adjacent the channel creates a bubble which causes a quantity of liquid ink to be ejected through an opening associated with the channel to the print sheet. The process of rapid vaporization creating a bubble is generally known as "nucleation." One patent showing the general configuration of a typical ink-jet printhead is U.S. Pat. No. 4,774,530, assigned to the assignee in the present application.
In ink-jet printing, it has been difficult to create an apparatus in which the size of a droplet ejected by a particular ejector can be selected. Generally, ejectors in thermal ink-jet printheads are capable of ejecting a droplet of generally one size only. However, there exist any number of printing situations where it would be desirable to be able to have a single ejector capable of selectably emitting a droplet of one of a plurality of selectable droplet sizes. Such situations in which a selectable droplet size would be highly useful include creation of half-tone images such as derived from photographs, and the creation of offset-quality alphanumeric characters.